1. Field of the Invention
This invention relates to an optical system for use in recording apparatus which has a plurality of light sources and records information in a parallel manner.
2. Description of the Related Art
An optical recording apparatus is conventionally known which uses optical means such as a laser beam to record information in concentrical or helical paths on a rotating disk-like information recording medium. With the advance and spread of information processing apparatus, such as computers, the information quantity to be dealt with expands. Accordingly, an optical recording apparatus is needed to shorten the access time for searching required information from recorded information and to increase the information quantity (transfer rate) which can be recorded in a unit time. To comply with this need, various improvements have been done to the optical head which performs the optical recording.
A known method of improving the transfer rate is disclosed in, for example, Japanese Patent Publications (Kokai) Nos. SHO59-65,948 and HEI1-312,747. In this method, a semiconductor laser array device emitting a plurality of laser beams is used to record and reproduce information in a parallel manner. Hereinafter, the configuration of this conventional example will be described with reference to the figures.
FIG. 1 is a plan view showing the optical system of the conventional optical recording apparatus. In the figure, 51 designates a semiconductor laser array device which emits four laser beams 52. FIG. 2 is a perspective view of the semiconductor laser array device 51. The four laser beams 52 are emitted from a single active layer 53. In the direction along which the semiconductor laser array device 51 emits the laser beams, a collimator lens 54, a beam splitter 55, and an objective 56 are sequentially arranged. The objective 56 forms four converged light spots 52A on the surface of an information recording medium 57. According to Japanese Patent Publication (Kokai) No. HEI1-312,747, the central two light spots among the four light spots are formed on a flat portion of the information recording medium 57, and the side two converged spots are formed on guide grooves of the medium 57. This arrangement is irrelevant to the object of the invention, and therefore in the following description it is assumed that four light spots 52A are respectively formed on four adjacent information tracks on the surface of the information recording medium 57. The four laser beams 52 reflected from the information recording medium 57 are reflected by the beam splitter 55, and then divided by a compound prism 58 into two groups. One group enters through a lens 59 an optical detector 60 which comprises at least four light receiving elements. In the direction along which the other group proceeds, a first prism 61 and a second prism 62 are arranged. Optical detectors 63 and 64 are disposed in the transmission directions of the reflecting planes 61A and 62A of these prisms, respectively. In the optical recording apparatus shown in FIG. 1, the portion consisting of the components other than the information recording medium 57 is generally called an optical head.
The operation of the apparatus will be described. The four light spots 52A are formed on the surface of the information recording medium 57 by the objective 56 so that they are respectively positioned on four adjacent information tracks. Using the four light spots 52A, therefore, information can be recorded in a parallel manner, thereby improving the transfer rate of information.
In order to record information in a parallel manner using the four light spots 52A, it is essential that each of the light spots is accurately positioned on the predetermined information track. For this purpose, the conventional apparatus uses the means which will be described below. Among the four laser beams 52 which have entered the first prism 61, because of the difference of incident angles, only one of the two side laser beams passes through the reflecting plane 61A and enters the optical detector 63. On the other hand, the remaining three laser beams are reflected from the reflecting plane 61A and enter the second prism 62. Similarly, because of the difference of incident angles, only the other one of the two side laser beams passes through the reflecting plane 62A and enters the optical detector 64. The two optical detectors 63 and 64 are two-segment optical detectors each comprising two light receiving elements, and perform a known tracking error detection method which is called the push-pull method, so as to judge whether or not the two side laser beams accurately follow the predetermined information tracks. If accurate tracking is not done, a moving mechanism (not shown) performs the correcting control so that the four light spots 52A are positioned in sequence on adjacent four information tracks.
The detection of reproduced signals is performed while the laser beams reflected from the information recording medium 57 are independently detected by the optical detector 60 which comprises at least four light receiving elements. According to this conventional apparatus, information can be recorded in a parallel manner using the four light spots, and therefore the transfer rate of information can be increased by four times.
As described above, the conventional optical recording apparatus is so constructed that the four laser beams 52 emitted from the semiconductor laser array device 51 are converted to parallel beams and then enters the objective 56. Therefore, the conventional apparatus has a problem as follows. This problem will be described with reference to FIGS. 3 through 5. FIG. 3 is a plan view of an optical system required for illustrating the problem of the conventional optical recording apparatus. In the figure, 54 and 56 designate the same components as those in FIG. 1, and the other optical components are not illustrated. The reference numeral 65 designates the optical axis of the optical system. In FIG. 3(A), S1 designates a light emitting point of the semiconductor laser array device 51 which is positioned on the optical axis 65. In this case, collimated beams 66 departing from the collimator lens 54 propagates in parallel with the optical axis 65 and enter the center of the objective 56. By contrast, in FIG. 3(B), S2 designates a light emitting point of the semiconductor laser array device 51 which is displaced by a distance H from the optical axis 65. In this case, collimated beam 67 coming out from the collimator lens 54 propagates obliquely with respect to the optical axis 65, and the central ray 67A of the collimated beams 67 enters the objective 56 with being displaced by a distance D from the optical axis 65, where the central ray 67A is defined as a ray proceeding parallel to the optical axis 65 between the semiconductor laser array device 51 and the collimator lens 54. When the focal length of the collimator lens 54 is represented by FC, the inclination U1 of the central ray 67A with respect to the optical axis 65 is given by the following expression: EQU U1=H/FC (rad.) (1)
When the optical path length between the collimator lens 54 and the objective 56 is represented by L, the deviation D of the central ray 67A from the optical axis 65 at the objective 56 is given by the following expression: ##EQU1## When L/FC is sufficiently greater than 1, the above expression can be rewritten as follows: EQU D=L.multidot.H/FC (3)
In the case where the semiconductor laser array device 51 is used as light sources, all the light emitting points other than at least one light emitting point are inevitably displaced from the optical axis 65, with the result that the deviation D of a substantial value is produced in each of the laser beams. The deviation D increases in proportion to the distance H between the light emitting point S2 and the optical axis 65 and also to the optical path length L. As the deviation D increases, the portion of the collimated beams 67 which enter the objective 56 decreases. This means that it becomes impossible to obtain a laser beam which provides the predetermined optical strength on the surface of the information recording medium 57. As a result, there arises a possibility that a reliable recording of information cannot be performed.
Generally, as shown in FIG. 2, a beam emitted from a semiconductor laser device has a for field intensity pattern whose beam divergence is narrow in the direction parallel to the active layer 53 (or lateral, hereinafter direction) and wide in the vertical direction. Beams emitted from a semiconductor laser array device have a similar shape intensity pattern as that described above. When a laser beam has an ellipsoidal shape intensity pattern as mentioned above, phenomena such as that also the light spot formed on the surface of the information recording medium 57 has an ellipsoidal shape and that a portion of the laser beam is eclipsed by optical components or the like are caused. The occurrence of such phenomena leads to the loss of the optical flux. In order to prevent such phenomena from occurring, therefore, a method is sometimes employed in which known means that is called as a beam reshaping is used for correcting the ellipsoidal shape intensity pattern of a laser beam to a substantially circular shape intensity pattern.
FIG. 4 is a plan view showing the main portion of an optical system and illustrating a problem which is caused in the same manner as FIG. 3(B) in a conventional optical recording apparatus using beam reshaping means. In FIG. 4, the reference numerals 54, 56 and 65 designate the same components as those in FIG. 3. The reference numeral 68 designates beam reshaping means which expands the divergence angle in the direction of the small width of a laser beam (hereinafter, referred to as "lateral direction") so as to substantially becomes equal to the divergence angle in the direction of the large width of the laser beam (hereinafter, referred to as "vertical direction"). An example of such beam reshaping means is disclosed in, for example, Japanese Patent Publication (Kokoku) No. SHO61-53,775. The beam reshaping means 68 is disposed immediately behind the collimator lens 54. In the same manner as FIG. 3(B), S2 designates a light emitting point which is displaced by a distance H from the optical axis 65. The collimated beam 69 coming out from the collimator lens 54 is not parallel with the optical axis 65. When the distance between the collimator lens 54 and the beam reshaping means 68 is represented by L1 and the inclination of the central ray 69A with respect to the optical axis 65 within that distance is represented by U1, the following relation is established: EQU U1=H/FC (rad.) (4)
The deviation D1 of the central ray 69A from the optical axis 65 at the beam reshaping means 68 is given by the following expression: ##EQU2## The inclination of the collimated beams 69 with respect to the optical axis 65 which have passed through the beam reshaping means 68 varies depending on the beam diameter expansion ratio M of the beam reshaping means 68. In this example, the beam diameter expansion ratio M is selected to be the ratio of the divergence angle in the vertical direction of the semiconductor laser device to the divergence angle in the lateral direction (hereinafter, the ratio is referred to as "the elliptic ratio"). When the inclination of the central ray 69B of the collimated beams 69 with respect to the optical axis 65 which have passed through the beam reshaping means 68 is represented by U2, the following relation is usually satisfied: EQU U2=U1/M=H/FC/M (rad.) (6)
Therefore, the deviation D2 of the central ray 69B with respect, to the optical axis 65 at the objective 56 can be given by the following expression: ##EQU3## where L2 is the distance; between the beam reshaping means 68 and the objective 56. When it is assumed that the distance L1 between the collimator lens 54 and the beam reshaping means 68 is equal to the focal length FC of the collimator lens 54, expression (7) can be simplified as follows: EQU D2=L2.multidot.H/FC/M (8)
When comparing the above expression with expression (3), it will be noted that, in the optical system which is provided with the beam reshaping means 68, the deviation of the central ray of a laser beam from a light emitting point which is displaced from the optical axis has a value obtained by dividing that in an optical system which is not provided with the beam reshaping means 68 by the beam diameter expansion ratio. Generally, the ratio of the divergence angle in the direction of the large width (vertical direction) to that in the direction of the small width (lateral direction) has a value from about 2 to about 4. Accordingly, the deviation in the optical system which is provided with the beam reshaping means 68 is reduced to one second or one fourth of that in an optical system which is not provided with the beam reshaping means 68. However, since the deviation varies in proportion to the optical path length L2, it is impossible to neglect the deviation unless the optical path length L2 is reduced to a sufficiently small value. As a result, there still remains a possibility that the laser beam fails to provide a predetermined optical strength.
The deviation of the central ray and the accompanying reduction of the power transmittance of an optical system will be described by illustrating a specific example. FIG. 5 is a graph showing the relationships between the optical path length and the central ray and optical system power transmittance in the optical system comprising the beam reshaping means 68. The abscissa is the optical path length L2 shown in FIG. 4, the left ordinate is the deviation of the central ray indicated by expression (8), and the right ordinate is the power transmittance of the optical system. The calculation conditions are listed below.
The divergence angle of the semiconductor laser beam in the lateral direction was 10 deg. (full angle at half maximum). PA1 The divergence angle of the semiconductor laser beam in the vertical direction was 30 deg. (full angle at half maximum). PA1 The distance H between the light emitting point of the semiconductor laser device and the optical axis was 0.2 mm. PA1 The beam diameter expansion ratio M of the beam reshaping means was 3. PA1 The focal length FC of the collimator lens was 7 mm. PA1 The focal length FO of the objective was 4 mm. PA1 The aperture radius RO of the objective was 2 mm. PA1 The power transmittance of the optical components was 100%.
In the calculation of the power transmittance of the optical system, it was assumed that the intensity profile of the beam of the semiconductor laser device is a 2-dimensional Gaussian distribution. When the light emitting point is on the optical axis, the conditions are the same as those in which the optical path length L2 is 0. In this case, the deviation of the central ray does not occur and the power transmittance of the optical system is 57.5%. In contrast, when the optical path length L2 is 100 mm under the conditions listed above, the deviation D2 of the central ray is 0.95 mm and the transmittance is reduced to 51.0%.
Hereinafter the above-discussed problem is summarized. In a conventional optical recording apparatus using a semiconductor laser array device, when a laser beam is emitted from a light emitting point which is displaced from the optical axis, a deviation between the central ray and the optical axis occurs at an objective. The amount of the deviation is in proportion to the distance between the light emitting point and the optical axis and also to the optical path length. Therefore, for a laser beam emitted from an outer light emitting point among light emitting points of the semiconductor laser array device, the loss of the light amount becomes greater. In an optical system which is provided with the beam reshaping means 68, the amount of the deviation is in inverse proportion to the beam diameter expansion ratio, and therefore the amount of the deviation may be reduced by increasing the beam diameter expansion ratio. When the beam diameter in the lateral direction is increased so that the beam diameter expansion ratio become greater than the ratio of the divergence angle in the vertical and lateral directions of the light source, however, the eclipse of an incident laser beam due to the objective increases, thereby causing a problem in that the loss of the light amount of the beam passing through the objective is caused. Furthermore, since the amount of the deviation is in proportion also to the optical path length, in an optical recording apparatus in which the optical path length cannot be shortened, there arises a big problem in that the loss of the light amount occurs in a laser beam whole light emitting point displaced from the optical axis.
Regarding the reduction of the access time, as disclosed in, for example, Japanese Patent Publication (Kokai) No. SHO60-239,943, a method is proposed in which a split type optical head is used or, among optical components constituting an optical head, the components required for the access operation are disposed in a moving unit and the other components are disposed in a fixed unit. This configuration can reduce the weight of the moving unit so that the access time is shortened.
FIG. 6 is a perspective view of a conventional optical recording apparatus in which a semiconductor laser array device and a split type optical head are combinedly used. Hereinafter, this conventional apparatus will be described.
In the figure, 1 designates a fixed unit of the split type optical head. The reference numeral 2 designates a moving unit of the split type optical head which can be moved by a driving mechanism (not shown) such as a linear motor along the radial direction (direction X shown in FIG. 6) of an information recording medium. In the fixed unit 1, the reference numeral 3 designates a semiconductor laser array device which emits a plurality of laser beams. The laser array device shown in FIG. 6 emits three laser beams. The reference numeral 4 designates a collimator lens which converts three laser beams emitted from the semiconductor laser device 3 to collimated beams, and 5 designates a beam splitter through which laser beams from the collimator lens 4 pass to be guided to the moving unit 2. In the moving unit 2, a reflecting mirror 6 and an objective 7 are arranged. Three laser beams which have passed through the beam splitter 5 pass the reflecting mirror 6 and the objective 7 in this sequence and are then converged on the information recording medium as three focused light spots 9 respectively formed on three adjacent information tracks 8. The three laser beams reflected from the information recording medium return to the fixed unit 1 and are reflected by the beam splitter 5 to be incident through a lens 10 on an optical detector 11. The reference numeral 12 designates a position detecting mechanism which consists of, for example, a linear scale and is disposed in the moving unit 2 of the split type optical head. The reference numeral 13 designates a moving unit position detection circuit which is connected to the position detecting mechanism 12, and 14 designates a laser power correction circuit which is connected to the moving unit position detection circuit 13. The reference numeral 15 designates a semiconductor laser device driving circuit which is connected to the laser power correction circuit 14, and the semiconductor laser device 3 is connected to the driving circuit 15 so that the three laser beams are independently driven. Although the apparatus of FIG. 6 has a three-beam semiconductor laser array device, any device which emits multiple beams may be used.
Next, the operation will be described. The semiconductor laser device driving circuit 15 independently drives the three laser beams of the semiconductor laser device 3. The three laser beams are converged by the objective 7 to be respectively incident on adjacent three information tracks 8 to form three converged light spots 9, so that each of the three laser beams is used for recording information. Therefore, the information recording can be performed at a triple transfer rate as compared with a conventional one-beam type optical head.
Among the optical components constituting the optical head, the optical components required for performing the access operation on the surface of the disk-like information recording medium, i.e., only the reflecting mirror 6 and objective 7 are separated from the other components to be disposed in the moving unit 2. Accordingly, the weight of the moving unit 2 can be decreased, thereby improving the access time. Reproduced signals from the three information tracks 8, focusing error signals indicative of the focusing error between the information track surface and the focused light spots 9, and track error signals indicative of the positional error between the information tracks 8 and the focused light spots 9 are detected by known detecting means. The operation and construction of such detecting means are irrelevant to the invention, and therefore their description is omitted.
Next, the method of driving the three laser beams of the semiconductor laser device 3 with a predetermined laser power will be described. In the split type optical head, since the position of the moving unit 2 or the position of the objective 7 varies in accordance with the access operation, the optical axis deviation D indicated by expression (2) described above varies depending on the position of the moving unit 2.
FIG. 7 is a development of a plan view of the optical system for illustrating the above description in more detail. In the figure, only the components required for description are shown. In the case that the distance L between the collimator lens 4 and the objective 7 is shortest, the moving unit 2 is located at position A which corresponds to the outer most track of the information recording medium. By contrast, in the case that the distance L between the collimator lens 4 and the objective 7 is longest, the moving unit 2 is located at position C which corresponds to the inner most track of the information recording medium. The position B of the moving unit 2 is an intermediate position between the positions A and C.
FIG. 8 is a graph showing the relationship between the position of the moving unit 2 in FIG. 6 and the laser power transmittance at the objective 7. The example shown in FIGS. 7 and 8 uses a three-beam type semiconductor laser device. In this example, the light emitting point of the center laser beam among those of the three laser beams is positionally adjusted to be located on the optical axis, and therefore the above-mentioned inclination U1 with respect to the optical axis 65 does not occur. As shown in FIG. 8, accordingly, the laser power transmittance of the center laser beam is constant irrespective of the position of the moving unit 2. In contrast, the light emitting points of the side two laser beams are inclined with respect to the optical axis 65. Since the optical axis deviation D due to the inclination with respect to the optical axis 65 varies in accordance with expression (2), the reduction of the laser power transmittance is maximum when the moving unit 2 is located at the position C. Even when the moving unit 2 is located at the position A, the optical axis deviation D occurs, and therefore the laser power transmittance of the side two laser beams is smaller than that of the center laser beam.
The relationship between the position of the moving unit 2 and the laser power transmittance which is shown in FIG. 8 depends on the distance H between the light emitting point S2 of the laser beam and the optical axis 65, the focal length FC of the collimator lens and the distance L between the collimator lens and the objective. However, the variation of the laser power transmittance of each laser beam will be made apparent by moving the moving unit 2 from the outer most track of the information recording medium to the inner most track after the step of assembling the optical recording apparatus.
Accordingly, in the configuration of FIG. 6, the position detecting mechanism 12 outputs a signal for detecting the position of the moving unit 2. This signal is processed by the moving unit position detection circuit 13 to be output as a signal indicative of the absolute position of the moving unit 2. In response to the variation of the laser power transmittance of each laser beam, the laser power correction circuit 14 corrects depending on the position of the moving unit 2 the preset value of the laser power, so that each of the laser beams maintains the predetermined recording power on the surface of the information recording medium. On the basis of the corrected value, furthermore, the semiconductor laser device driving circuit 15 drives the semiconductor laser device 3 to control the power of each laser beam. Irrespective of the position of the moving unit 2, therefore, each laser beam can maintain the predetermined laser power on the surface of the information recording medium.
In the conventional optical recording/reproducing apparatus having the above-mentioned configuration, it is necessary to accurately detect the position of the moving unit 2, and the position detecting mechanism 12 and moving unit position detection circuit 1 are provided for this purpose. In order to correct the variation of laser power depending on the position of the moving unit 2, the laser power transmittance must be previously measured in the step of assembling the optical recording apparatus, and the laser power correction circuit 14 must be provided which functions so as to maintain the laser power transmittance at a predetermined value. These requirements cause the size of the apparatus to be increased and further the assembling time to be lengthened, thereby increasing the manufacturing cost.
In the conventional apparatus, the laser power is corrected on the basis of the position of the moving unit 2 and laser power transmittance at this position which are previously measured in the step of assembling the apparatus. Therefore, there is a problem in that, when the time-varying variation of the laser power transmittance due to the deterioration of the semiconductor laser device 3, dust in the optical system, etc. occurs, it is impossible to irradiate the information recording medium with a required laser power and therefore the recording of information cannot be stably performed.